Frequently in the television industry it is desirable to convert motion pictures or other image media such as video tape into video signals for recording or for transmission of the images to a viewing audience. Many technical problems are encountered in converting the motion picture images into video signals, including problems in the spectral response of the color dyes in the film, the characteristics of the telecine, and the fact that the characteristics of the photographic film are not matched to the electronic components used in the telecine or "film chain". Especially, problems are encountered in insuring that the spectral content of the resultant video signals are aesthetically acceptable and that the luminance or monochrome portion of the video signal provides an acceptable black and white image when reproduced on monochrome equipment.
One proposal for a color correction system for use in a film chain is shown in U.S. Pat. No. 4,096,523 to Belmares-Sarabia et al. In this system, color corrections are made in the video signals by adjusting the color hue and saturation, and by varying the proportions of the primary color video signals to form a composite luminance signal. The entire film is color-corrected by providing incremental adjustment signals which are added to or subtracted from standard values for each different scene in the film. The incremental adjustment values are stored in a computer memory associated with an event number, which is in turn associated with the count of the frame in the film at which the color correction was made.
After a series of color corrections have been made, a video tape recording of the color-corrected video signals is produced by re-running the motion picture film, video tape, or other video media through the system, while the color correction incremental adjustment signals are recalled from memory and added to the signals produced during the running of the film. The resultant color-corrected composite video signals are then recorded on the video tape recorder.
One problem with the system shown in U.S. Pat. No. 4,096,523 is that in creating the corrected video signals and the corrected composite luminance signal, control is provided only over the primary color video signals. Accordingly, if a spectral content correction or a luminance correction is effectuated by adjusting one of these primary video channels, any color mixtures in the video scene of which that primary color video signal is a primary color component together with another primary color component, will also be affected.
For example, with respect to luminance correction, signals designated R, B and G from the video pick-up devices are processed and provided to a luminance control circuit. For each of the processed signals, designated PR, PB, and PG, there is provided a video amplifier which is controlled by a DC signal from a remote luminance control potentiometer. A conventional luminance signal, developed by a resistor network in accordance with the NTSC standard luminance mixture in the proportion of 30% of red, 11% of blue, and 59% of green, is then modified by controlling the PR, PB, and PG video amplifiers. Accordingly, it is apparent that in this device the luminance potentiometers separately vary the total red signal in the red channel in order to vary the amount of the red signal away from the standard 30%. Similarly, other luminance potentiometers provide for adjustment of the green and blue channels.
Inasmuch as the total red channel in the apparatus shown in U.S. Pat. No. 4,096,523 is luminance controlled, it follows that any portions of a video picture containing red information will be affected in luminance by adjustment of the luminance potentiometer for the red channel. For example, in a video scene having a red soft drink cup, an orange fruit, and a teak desk, each of these objects will vary in luminance or brightness by adjusting the luminance potentiometer for the red channel, since each of the objects of the picture contains some red information. Adjusting the red luminance potentiometer to increase the amount of red in the overall luminance signal will make brighter any object in the scene which contains any red. The red cup will be most affected, since it is primarily red, while the orange and the teak desk will also be affected, although somewhat lesser, since these objects have colors which include red in some amounts as one of the primary color components.
Accordingly, if the control in the red channel is adjusted, the luminance of any object in the scene, the color of which includes red as one of the primary color components, will be altered. Neutral tones such as whites and grays will also be affected. It is therefore apparent that the color correction device shown in U.S. Pat. No. 4,096,523 undesirably affects the spectral content of neutral tones and color mixtures containing one of the primary colors as one of the color components, when the luminance of that particular primary color is adjusted.
One proposal for providing a greater control over video signals is shown in U.S. Pat. Nos. 3,558,806 to Monahan et al. and 4,410,908 to Belmares-Sarabia et al. In particular, the latter patent employs the circuitry of the former patent in order to obtain six independently variable primary and complementary color derivative signals (yellow, green, red, blue, cyan, and magenta), which are combined with one another to obtain a luminance compensation signal. In essence, these patents disclose color separator circuitry which is responsive to the primary color video signals to provide a plurality of independent color derivative signals, each of which is separately variable so as to provide control over parameters such as luminance, hue, or saturation. Each of the independent color derivative signals is present only when the video image color information is predominantly the color of the particular independent color video channel.
Present day users of video color correction equipment desire even greater precision and control over video parameters such as hue, saturation, and luminance than can be provided with the apparatus disclosed in the Monahan and Belmares-Sarabia patents. For example, consider extending the concept shown in these patents to provide for independently variable color derivative signals other than the six primary and complementary colors. For each additional color derivative signal, there will necessarily be required a separate additional color separator circuit responsive to predetermined proportions of signal information present in the primary video channels, so that the presence of color information predominantly the color of the separated color channel can be detected for purposes of exercising control. Accordingly, it is apparent that if control is desired over eight, or ten, or twelve different colors, as opposed to the six primary and complementary colors, there will be needed eight, or ten, or twelve additional channels for developing the independent color derivative signals.
Accordingly, there is a need for color video control circuitry which is able to detect and control parameters of color video signals without requiring a separate channel of circuitry for each color over which control is desired to be exercised.